[clangd] Disable expand auto on decltype(auto)

Summary: Applying it produces incorrect code at the moment.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: kuhnel, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D68630

llvm-svn: 374048